In a disc apparatus, such as CD player which plays back a compact disc (CD) having concentric tracks on which digital audio data are recorded as a pit train, the disc is irradiated with a laser beam along the tracks thereon while the disc is driven to rotate at a constant linear velocity by a spindle motor. The digital audio data are reproduced by detecting changes in the strength of the reflected light due to the presence or absence of the pits.
Although the error rate on reproduction of the data may be, for example, about 10.sup.-5 in the CD player, the reproduced data are subjected to an error correction processing using error detection codes and error correction codes so that no problem will occur in the normal application environment.
In a CD player, which includes an optical reproducing head, a servo system, such as a focus servo or a tracking servo for the reproducing head, will deviate due to mechanical disturbances such as vibration or shock so that normal data reproduction may become difficult. In such a case, an error can not be corrected even if the above mentioned error detection codes or error correction codes are used, so that reproduction may be temporarily interrupted.
In a CD player for vehicles or a portable CD player, which have a high possibility of experiencing a large vibration or shock, unlike a desk top type home use CD player, a mechanical anti-vibration mechanism has heretofore been provided to prevent the servo system from deviating due to disturbance as mentioned above.
In the so-called CD-I (CD-interactive) system for simultaneously recording video data, character data, and audio information on the compact disc, 6 modes have heretofore been standardized in addition to a CD digital audio (CD-DA) mode as shown in FIG. 6.
Linear PCM (Pulse Code Modulation) having a sampling frequency of 44.1 kHz and a quantization number of 16 bits is used in the CD-DA mode having a level of sound quality equivalent to existing 16 bits PCM. ADPCM (Adaptive Differential Pulse Code Modulation) having a sampling frequency of 37.8 kHz and a quantization number of 8 bits is used in the A level stereo mode and A level monaural mode having a sound quality equivalent to an LP disc record.
ADPCM having a sampling frequency of 37.8 kHz and a quantization number of 4 bits is used in the B level stereo mode and the B level monaural mode having a sound quality equivalent to FM broadcasting. ADPCM having a sampling frequency of 18.9 kHz and a quantization number of 4 bits is used in the C level stereo mode and C level monaural mode equivalent to AM broadcasting.
In other words, as shown in FIG. 6, the bit saving factor is 1/2 in the A level stereo mode in comparison with CD-DA mode. Data are recorded at intervals of 2 sectors. The black squares in the drawing represent recorded sectors. The reproduction or playback period of time of one disc is about 2 hours. The bit saving factor is 1/4 in the A level monaural mode. Data are recorded at intervals of four sectors. The reproduction period time is about 4 hours. The bit saving factor is 1/8 in the B level monaural mode. Data are recorded at intervals of 8 sectors. The reproduction period of time is about 8 hours. The bit saving factor is 1/8 in the C level stereo mode. Data are recorded at intervals of 8 sectors. The reproduction period of time is 1/16 in the C level monaural mode and data are recorded at intervals of 16 sectors. The reproduction period of time is about 16 hours.
For example, in the B level stereo mode, the audio information is discretely recorded in sector units at intervals of 4 sectors along the tracks from the first sector on the innermost track.
After the audio information is recorded on the outermost track, the audio information is recorded along tracks at intervals of four sectors form the second sector on the innermost to the outermost tracks. When the audio information which has been recorded in such a manner is reproduced, the data is not continuously reproduced so that playback music, for example, is interrupted while the playback head jumps to return from the outermost track to the innermost track.
A disc recording apparatus which records digital data which meets the standards of the above mentioned CD or CD-I on an optical disc, of a type which will be described hereafter, or an information rewritable magneto-optical disc has heretofore been provided. Also in this disc recording apparatus, the servo system of a focus servo or tracking servo for a recording head will deviate due to mechanical disturbances such as vibration or shock so that recording may be temporarily interrupted.